Total Cost of Ownership Analysis?

[Brian Noble]

However, it should be noted that no consideration was made for capital gains or ordinary income taxes (dependent upon individual situation) on the investment which would effectively reduce overall ROI and likely produce a shorter life on the investment.

I think Mary's analysis can be considered to account for that by treating all cash flows as after-tax amounts. Then, if you pick the opportunity cost rate to be your expected after-tax rate of return on investment, then you're comparing all amounts on the same basis. Her 7% figure isn't a bad one---it's a pretty close match to the long-term after-tax return one would expect from an S&P 500 index fund.